Have you ever owned a Citroen 2CV or a VW Beetle? How about those sheet metal boxes around the hot bits of the aircooled cylinders and exhaust connections? Yeah, they are a cheap solution to providing heated air to the cabin. Any chance of an exhaust gasket leak, or fumes from seeping oil heated at n degrees on the cylinder head actually being sucked into the cabin. "Slim to none, let's go with it. Otherwise, geez, you're talking a real costly solution."
Bypass air straight to the cabin? What were they/we thinking?